Structural analogs of the peptide hormone, Angiotensin-II, are being prepared for biological evaluation. Amino acid side chains are being altered in a systematic fashion to define their role in the production of the biological responses. Contraction of the guinea pig ileum and increase in blood pressure in the rat are used as the biological test systems in studies for agonist and antagonist response. Conformational studies using nuclear magnetic resonance, optical rotatory dispersion, and circular dichroism spectroscopy are carried out on selected analogs.